I think that Yanmars...
... are good engines, but basically cold hearted beasts. I've got a 2GM20F in my boat that starts and runs well for a 20 yr old engine. When it starts to get cold around Deale (like October....), my engine gets a bit more difficult to start, perhaps like yours. Assuming all the obvious is ok (i.e., good fuel, clean filters, no corrosion in the wiring harness, etc.), what I do is help the engine a bit on cold days. I plug in a small electric hair dryer and aim it at the air intake. The engine sucks in nice warm air and thinks its 80-deg outside and starts right up. Give it a try. PS: If you are cranking your engine for more than 30 seconds or so, close the raw water intake first to avoid water filling your muffler and backing into the engine. Remember to open it as soon as the engine fires... Also, another possible problem could be that your exhaust elbow is coking up and not allowing the engine to breathe. It may have to be replaced.